The present invention relates to a method for improving pad bonding of an IC, and in particular to a method of improving the pad bonding of an IC by removing diffusion barrier layer on the pad area of a VLSI circuit.
In the fabrication of VLSI circuits, a diffusion barrier layer, such as Ti/TiN or Ti/Ti:W, has been widely introduced between aluminum alloy and silicon contact to avoid the abnormal interfacial diffusion and increase the circuit lifetime. However, it is easy to form an unstable compound of TiSixOy between Ti layer and underlying dielectric, e.g. BPSG (Boro phospho silicate glass). This unsatable compound shows poor adhesion on underlying BPSG and always result in bad bonding yield on the connection from circuit pad to package frame.
In order that the disadvantage of the above-mentioned conventional technique can be better:understood, the process of fabricating conventional VLSI circuits is described hereinafter with reference to FIGS. 1a to 1e.
Referring now to FIG. 1a, a Si substrate 10 is laterally isolated with a field oxide 12 with a thickness of 3000 to 8000 A.degree. by using for example LOCOS (Local Oxidation of Silicon) technology so that an active region is formed on the Si substrate 10. The isolation region, i.e. the field oxide 12, may be formed by other conventional methods understood by those skilled in this art, for example by the trenched dielectric method. After the isolated field oxide 12 is formed, a doped polysilicon or polycide is deposited on a gate oxide 14 to a thickness of 1500 to 4000 A.degree. which is then etched to form a gate elective 16. Thereafter, the N+ (or P+) source/drain regions 18 are formed by implanting with As+ or P+ for N+ and BF.sub.2.sup.+ or B+ for P+ to a dose of between about 1E14 to 1E16/cm.sup.2 and energy of between about 10 to 200 KeY, followed by a high temperature activation. In some VLSI circuits, lightly doped drain(LDD) or double diffused drain(DDD) structures or the like are used.
The next step can be seen with reference to FIG. 1b. The FIG. 1a transistor structure is then deposited to form a premetal dielectric SiO.sub.2 layer 20, typically Borosilicate glass(BSG) or Borophosphosilicate glass(BPSG) with a thickness of between about 3000 to 10000 A.degree. followed by a high temperature densification. The densification involves placing chips in a high temperature ambient to make the deposited premetal dielectric SiO.sub.2 layer 20 densified and stable. Metal contact windows 22 are then formed on the densified premetal dielectric SiO.sub.2 layer 20 by dry or wet etching.
The next step is shown in FIG. 1c wherein diffusion barrier layers TiN 24/TiSi.sub.2 26 are formed on the metal contact windows 22, and TiN 24/TiSixOy 28 layers are formed on the premetal dielectric SiO.sub.2 layer 20. The formation of diffusion barrier layer is completed by sputtering Ti, followed by rapid thermal annealing in a nitrogen ambient. Alternatively, the diffusion barrier layer can be formed by sputtering a bi-layered structure of Ti and TiN, or Ti and Ti:W, followed by the same kind of rapid thermal annealing.
Referring now to FIG. 1d, on the diffusion layers are then deposited with a metal layer 30, usually an Al alloy such as Al-Si(0-2%)-Cu(0.5-4%) with a thickness of about 4000 to 10000 A.degree.. The metal layer 30 is then patterned using conventional lithography and etching techniques to form the patterns as shown in the drawing.
The final series steps involve depositing a passivation layer 32, typically SiO.sub.2 and Si.sub.3 N.sub.4 layers with a total thickness of about 5000 to 20000 A.degree. by Chemical Vapor Deposition(CVD), patterning and then forming a pad area 34 by masking and etching the passeration layer 32. The finished VLSI circuit is shown in cross section in FIG. 1e.
According to the above conventional technique for fabricating VLSI circuits, the presence of the diffusion barrier layers is used to get rid of aluminum spiking and silicon precipitate on contacts. Al-Si-Cu alloys are used as the metal layer 30 for the purpose of solving the aluminum spiking problem. However, using Al-Si-Cu alloys results in an excessive amount of Si precipitation on the surface of the Si substrate 10 underlying the metal contact windows 22, thus increasing the contact resistance thereof. Moreover, with the scaling down of the device dimensions in VLSI circuits, it is desired to increase the stepcoverage of the metal layer of metal contact Windows 22, and thus a higher sputtering temperature may be used. This will worsen the silicon precipitation problem. In order to solve the above problems, diffusion barrier layers of TiN 24/TiSi.sub.2 26 are formed on metal contact windows 22. However, this step also forms a unstable TiSixOy layer 28 on premetal dielectric SiO.sub.2 layer 20. As this unstable layer 28 exhibits poor adhesion to the underlying premetal dielectric SiO.sub.2 layer, it results in bad pad bonding yields.